More than 150 recombinantly produced proteins and polypeptides have been approved by the U.S. Food and Drug Administration (FDA) for use as biotechnology drugs and vaccines, with another 370 in clinical trials. Proteins tested to date come from both prokaryotic and eukaryotic sources and are quite varied in both structure and function. Optimizing expression for a wide variety of proteins involves testing and usage of a multitude of factors which can affect transcription, translation, solubility and stability of the protein of interest. Factors which can affect protein expression are environmental (eg. temperature or nutrients), host cell specific (eg. protease deficiency or chaperone overexpression) or plasmid specific (eg. type of promoter or plasmid copy number).
One means to increase gene expression involves increasing the number of copies of the plasmid from which the gene is transcribed, either by increasing the number of copies of the cloned gene within each expression plasmid or by increasing the copy number of the plasmid on which the gene to be expressed resides. It is known that plasmids must control their replication so that the copy number (N) of a given plasmid within a population of cells is usually maintained within a narrow Gaussian distribution within a given host and under defined growth conditions. This control is required, since plasmids must co-exist stably within their hosts and minimize metabolic load upon the cell. Specifically, over-accumulation of plasmid copies within a cell can slow cell growth and eventually cause cell death.